In order to pump oil from a well in which casing has been set, the casing is perforated by a suitable gun at those levels at which oil has been indicated to exist. After the casing has been perforated, a pump is lowered on the end of the conventional drill pipe string to this particular level. The pump includes a pumping tube hereafter referred to as a pump barrel within which a reciprocating plunger operates. This plunger is driven by a polish rod extending upwardly through the tubing to connect to the end of sucker rods which pass up through the drill pipe string to the surface of the well. The term "oil well tubing" as used herein is meant to include the pipe string and pump barrel. Oil is pumped from the lower annulus adjacent the perforations in the casing through the well tubing to the surface of the well.
After pumping has been completed, or if it is desired to change the pump or repair it, it is necessary to pull the tubing to bring the pump to the surface. Since, however, the oil well tubing is filled with oil throughout the length of the tubing including the pipe string, whereas the surrounding oil in the well annulus defined between the exterior of the well tubing and interior of the casing is at a relatively low level, there exists a very large pressure differential rendering it extremely difficult to pull the tubing.
The foregoing condition can be overcome if the fluid or oil within the well tubing could be passed in a reverse direction to the annulus exterior of the well tubing. If the fluid could be passed in this manner, the large hydrostatic head created within the well tubing could be "dumped" into the surrounding annulus of the well and thus equalize the pressures involved so that removal of the tubing without the large hydrostatic head of oil could be readily achieved.
My U.S. Pat. Nos. 3,168,873 issued Feb. 9, 1965 and 4,047,853 issued Sept. 13, 1977 disclose hydraulic unloading and circulating devices which solve the foregoing problem. However, the solutions described in these patents are not suitable for certain types of oil well pumps.
More particularly, there has been introduced an improved type of oil well pump capable of delivering a greater capacity of oil through a pipe string than previously available pumps. One such improved type pump is provided by the Heavy Oil Producer Service, Inc. referred to in the art as HOPS type pumps. Pumps of this type include a relatively large diameter pump tube which connects to the extreme end of the drill pipe string rather than being received within the drill pipe string. For such a pump, it would be desirable to incorporate some type of hydraulic unloading or pressure equalizing means in or as a part of the pump barrel itself. Further, a desirable feature is to be able to operate the "dumping" or equalizing system from the surface of the bore hole. Moreover, such system should be designed so that it will provide continuous communication between the interior of the oil well tubing and the annulus while pulling the tubing. would be provision of such a hydrostatic equalizing means which could be incorporated in the newer type referred to pumps with a minimum of structural modification, all to the end that overall economy is realized in the manufacture and use of the device.